1. Field of the Invention
The present invention relates to an apparatus for transmitting information signals between a camera and its accessories such as a taking lens. More particularly, the invention relates to an apparatus useful in transmitting lens displacement information from a taking lens unit to the body of a camera equipped with a focus detecting mechanism.
2. Description of the Prior Art
In single-lens reflex cameras with an auto-focusing mechanism, the about of light passing through a taking lens is determined by a TTL focus detecting device which, on the basis of the determined amount of light, detects the deviation between the predetermined focal plane of the taking lens, for example, the film surface, and the plane in which the actual image of the object is formed. Typical examples of this TTL focus detecting device are shown in U.S. Pat. Nos. 4,185,191 and 4,264,810. The taking lens can be brought into focus by driving the focusing lens in the taking lens system by the amount corresponding to the detected deviation of image plane. If the taking lens has a fixed focal length, the distance the focusing lens must be moved has a substantially constant relation with the detected deviation of image plane. Therefore, precise focusing can be achieved by simply collating the required displacement of the focusing lens against the detected image plane deviation. But with a taking lens such as a zoom lens having a variable focal length, the relation between the distance the focusing lens must be moved and the detected deviation of image plane also varies with a variation in the focal length. Therefore, precise or rapid auto-focusing is difficult to achieve with a taking lens having a variable focal length.
To ensure precise or rapid auto-focusing in a single-lens reflex camera, a plurality of information signals must precisely be transmitted between the camera body and the interchangeable taking lens unit. In addition, modern cameras require that various pieces of information such as aperture value, object distance and minimum aperture value be transmitted from the lens unit to the camera body as electrical signals. For this purpose, a number of terminals are necessary, but it is difficult for the lens unit to provide for a space large enough to accommodate these terminals. Furthermore, the chance of failures such as poor contact will increase as more terminals are used. We already filed U.S. Ser. No. 441,787 on Nov. 15, 1982 and proposed a technique for eliminating these problems, but a further improvement is desires.